Magnetic anomaly detector for detecting the movement of ferrous metals

ABSTRACT

A magnetic anomaly detector for detecting the presence of ferrous metal objects such as firearms, knives, or other weapons. The detector contains a square wave generator, a sensor for sensing ferrous metal objects, a sensor amplifier, a main amplifier, and a display. When a ferrous metal object is detected by the sensor, the resultant anomaly in the magnetic field causes portions of a pulse train being input to the sensor amplifier extend past a preset threshold, causing an average input value of an integrator to change. The integrator then forces the average input value of the waveform to zero over an extended period of time to compensate for the changes in the magnetic field. The value at the integrator input represents the magnetic anomaly. The anomaly is also visually displayed in an LED bar graph display for easy detection of weapons by the operator.

This application claims benefit of provisional application No.60/030,781 filed Nov. 13, 1996.

TECHNICAL FIELD

The present invention relates to magnetic anomaly detectors for use inmetal detectors, and more particularly to a magnetic anomaly detectorthat senses changes in the earth's magnetic field caused by movement offerrous material near the detector.

BACKGROUND OF THE INVENTION

Many buildings use metal detectors to screen incoming visitors forweapons such as firearms or knives. These detectors are most commonlyseen at airports, but the use of metal detectors in schools andcourthouses is increasing due to rising concern about the possibility ofviolent crime in these institutions.

A common problem with conventional metal detectors is that they cannotdistinguish between objects containing iron ("ferrous") and those thatdo not contain iron ("non-ferrous"). The vast majority of weapons aremanufactured from ferrous metals, but since many metal detectors willdetect non-ferrous metals as well as ferrous ones, the detectors willsound an alarm in the presence of many common metallic objects, such askeys, belt buckles and coins, in addition to weapons. Since most peopleentering a building would be carrying keys or coins, conventional metaldetectors tend to sound an alarm for nearly every person walking throughit, requiring security officers to either ask the person to remove thekeys or coins from his or her pockets or use a separate metal detectingwand to pinpoint the location of the offending metal objects. Bothalternatives tend to be time-consuming and somewhat of an inconvenience,particularly if there are many people waiting in line to pass throughthe metal detector.

Most metal detectors use a "beat frequency oscillator" ("BFO") detector.This type of detector has a search loop oscillator and referenceoscillator. These two oscillators oscillate at the same frequency ifthere is no metal near detector. When a metallic object passes throughthe BFO detector, the search loop oscillator frequency changes inrelation to the frequency of the reference oscillator. This causes thebeat frequency between the search loop oscillator and the referenceoscillator to increase, setting off an alarm. The main problem with theBFO detector is that it will detect metallic materials indiscriminately;it does not distinguish between ferrous metals (which are most commonlyused in weapons) and other metallic substances, such as those used incoins and keys.

One possible method of distinguishing between ferrous and non-ferrousmetals is by using a magnetometer, such as the one in U.S. Pat. No.5,432,445 to Dinsmore et al, entitled "Mirror Image DifferentialInductor Amplitude Magnetometer". Since only ferrous metals aremagnetic, magnetometers can distinguish between ferrous and non-ferrousmetals. The sensor shown in Dinsmore et al. requires an outside coilwrapped around a pair of closely matched coils for sensing the magneticfield. Each of the matched coils has a magnetically permeable strip inits center. The matched coils must be identical and the magneticcharacteristics of the center strips must also be identical, or a"mirror image" of each other. Manufacturing the center strips isparticularly cumbersome and expensive; since each of the strips ispunched from a single sheet of metal, the mechanical stresses from thepunching process changes the magnetic characteristics of every strip.The strips must then be heat-treated through a hydrogen annealing methodin an attempt to bring the strips' magnetic properties back to theiroriginal state. However, it is impossible to bring the magneticproperties of each individual strip to exactly the same state as beforethe punching process or to match the magnetic properties between strips.Since the matched coils and center strips must be "mirror images" ofeach other, any mismatch in the characteristics between the coils andbetween the strips, no matter how slight, will render the sensor, andthus magnetometer, useless. In initial batches of sensors tested by theinventor, only about half of the finished sensors were usable.

In addition, the circuitry in the Dinsmore et al. magnetometer requiredan extremely pure sine wave input to drive the sensor. Generating a purestable sine wave input without any changes in its frequency or amplitudeis difficult and unduly complicates the circuitry within themagnetometer. The sensor generates two outputs, requiring manymanipulations of the two outputs in the signal conditioning in theDinsmore et al. magnetometer to generate a voltage which is proportionalto the magnetic field sensed by the sensor, thus further complicatingthe circuitry. This circuit could also be used to detect minute changesin the earth's magnetic field, as opposed to the absolute value of theearth's magnetic field, but additional complicated circuitry is neededand a large output voltage is needed to make the device more sensitive.However, increasing the output voltage in a DC magnetometer oftensaturates the circuit if the field is too strong, making detection ofany changes in the magnetic field impossible since the value of theoutput voltage is too close to the value of the power supply.

OBJECTS AND ADVANTAGES

Accordingly, it is an object of the invention to create a magneticanomaly detector which can measure minute changes in the earth'smagnetic field while ignoring DC changes unrelated to the movement offerrous materials, such as .the orientation of the sensor, steel beamsin the building, or nearby steel furniture.

It is another object of the invention to provide a magnetometer having asensor which does not require any mirror imaging of any of its internalparts.

Further objects and advantages will become apparent from a considerationof the following description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the magnetic anomaly detector of thepresent invention;

FIGS. 2A through 2D are diagrams of a sensor in the magnetic anomalydetector;

FIG. 3 is a schematic of one embodiment of a sensor amplifier in themagnetic anomaly detector;

FIGS. 4A through 4G are waveforms taken at various test points in thesensor amplifier of FIG. 3;

FIG. 5 is a schematic of an alternative sensor amplifier in the magneticanomaly detector;

FIGS. 6A through 6D are diagrams showing the operation of a selectiveintegrator in the alternative sensor amplifier shown in FIG. 5;

FIG. 7 is a schematic of yet another alternative sensor amplifier in themagnetic anomaly detector;

FIG. 8 is a schematic of a main amplifier in the magnetic anomalydetector; and

FIG. 9 is a schematic of a display board in the magnetic anomalydetector.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates the basic components of the magnetic anomalydetector. FIG. 1 shows two sensors 100, each connected to a sensoramplifier 200, 300 or 400. The outputs of the respective sensoramplifiers 200, 300 or 400 are fed to a main amplifier 500. The sensoramplifier 200, 300 or 400 not only provides a drive signal to the sensor100, but also signal conditions the output from the sensor 100 so that asmall change in the magnetic field strength detected by the sensor 100results in a large, easily discernible change in the output voltage ofthe sensor amplifier 200, 300 or 400. The main amplifier 500 firstamplifies and fullwave rectifies the two inputs, then sums them in asumming amplifier. The summing amplifier stage in the main amplifier 500provides a sensitivity adjustment and an adjustable voltage reference toeliminate the effects of variations in the earth's magnetic field itselfon the output result. The last stage in the main amplifier 500 is a peakdetector with bleed resistor. This peak detector enables LED's in adisplay 600 to stay on longer so they can be read easily. The display600 converts a voltage to a 10 LED bar graph display and includes abuzzer with an adjustment means so that when the bar graph LED's reach apredetermined value, the buzzer will activate.

FIGS. 2A through 2D illustrate how the sensor 100 is constructed.Referring to FIG. 2A, the sensor 100 contains a plastic bobbin 110having three copper leads 112, 114 and 116 attached to it. The leads arepreferably made of non-ferrous metal so that the leads will not distortthe magnetic field near the sensor. FIG. 2B shows a high permeabilitythin metal alloy strip 120 attached to the bobbin on one side with anadhesive. This metal strip 120 is made of highly permeable material suchas MU metal, Advanced Magnetic AD-90 by Advanced Magnetic Corporation.The cross sectional area of this strip must be very small so thatsaturation can easily occur when the sensor 100 is driven by an externalcoil. The metal strip 120 can not be cut mechanically because thethickness of the metal is very thin. If metal strips are cut bymechanical means, the magnetic properties of the strip will be lost.Although hydrogen annealing can partially recover the magneticproperties of the strip, it still will not return the magneticproperties of the metal to its original state.

Instead of mechanical cutting, the metal strips 120 of the presentinvention are preferably cut from a large virgin material sheet by achemical etching method. First, both sides of the thin high permeabilitymetal sheet are covered with photoresist and dried in an oven. Next, twoidentical photo positive films having an image of many strips is used tosensitize the metal sheet. After the metal sheet is sensitized, theleftover photoresist is removed and the entire sheet is cleaned. Thesheet is then dipped into acid for etching. Since the dividing linesbetween the strips were exposed and the strips were protected in thesensitizing process, all the strips will be separated from each otherafter the etching process. Each sheet can yield hundreds of metal strips120 without destroying the magnetic properties of the virgin materialsince the sheet undergoes no mechanical stresses.

Referring to FIG. 2C, a winding 130 is wound on top the combination ofthe bobbin 110 and the magnetic strip 120. This winding will terminateat terminal 112 and terminal 114. A second winding 140 is wound on topof the first winding 130, as shown in FIG. 2D. This second winding 140will terminate at one end at terminal 112 and the other end at terminal116. After this step, a layer of protective coating is applied toprotect the windings 130 and 140. The completed sensor assembly formssensor 100.

FIG. 3 shows one embodiment of sensor amplifier 200. Sensor amplifier200 converts minute changes in magnetic flux disturbance caused by themovement of ferromagnetic metal near sensor 100 into large voltagechanges at the output of the sensor amplifier 200. A square oscillator202 contains a 555 timer IC1, resistor R1 and capacitor C1, which areconnected together to generate a near 50% duty cycle square wave. Theoutput amplitude of the oscillator 202 in this example is 0 to 5 volts,as illustrated in FIGS. 4A at test point 1.

A DC de-coupling stage 204 contains a blocking capacitor C3 and resistorR2. The DC decoupling stage 204 is used to block out the DC voltage inthe waveform output from the oscillator 202 and prevent it from reachingthe next amplifier stage. The output of DC decoupling stage 204 in thisexample is a square wave having a magnitude between -2.5 V and +2.5 V.

A driver stage 206 connected to the DC decoupling stage 204 has anoperational amplifier IC2 and a pair of complementary transistors Q1 andQ2. The driver stage 206 is connected as a unity gain buffer andprovides the current needed to drive the sensor 100. Refer to FIG. 4B,which shows the output waveform of the driver stage 206. A resistor 208is used to limit the current flowing to sensor 100.

An adjustable gain amplifier 210 connected to the sensor 100 contains anoperational amplifier IC3 , resistors R5, R6, R8, potentiometer R7, andcapacitor C4. The adjustable gain amplifier 210 amplifies the output ofthe sensor 100. Capacitor C4 and resistor R5 are used to separate the ACsignal from DC feedback that shared the same winding of sensor 100. FIG.4C shows the pulse train at the input of the amplifier 210. Operationalamplifier IC3 in the adjustable gain amplifier 210, along with resistorsR6, R8 and potentiometer R7, are connected together as a non-invertingamplifier with gain adjustment capability.

Selective amplifier 212 contains Zener diodes ZD1, ZD2 resistors R9,R10, R11, operational amplifier IC4 and capacitor C5. The selectiveamplifier 212 amplifies only the portion of the pulse train which iseither larger than Vr or smaller than -Vr; in other words, the selectiveamplifier only amplifies the extreme tips of the pulse train outside ofthe -Vr/Vr range. Vr is the sum of the breakdown voltage plus theforward drop of the Zener diode ZD1 or ZD2. For example, if the Zenerdiodes used in the selective amplifier 212 is a 1N751, it will have abreakdown voltage of 5.1 V and a forward drop at low current levels ofapproximately 0.3 V. Thus, the selective amplifier 212 in this examplewill amplify only the part of the pulse train which is higher than 5.4 Vor lower than -5.4 V. If the pulse train does not extend outside of the-5.4 V/5.4 V range, the output of the selective amplifier 212 will stayat zero level. Resistors R10 and R11 are used to set the gain of thenon-inverting amplifier in the selective amplifier 212 and capacitor C5sets the bandwidth and provides low pass filtering and pulse stretching.FIG.4D shows the output waveform of the selective amplifier 212.

Next, an inverting summing amplifier 214 contains operational amplifierIC5 , resistors R12, R13, R15, capacitor C6 and potentiometer R14. Theinverting summing amplifier 214 provides additional gain and filteringto the system. Also, the potentiometer R14 and resistor R13 compensatesfor the mismatch of the breakdown voltages between the two Zener diodesZD1 and ZD2 in the selective amplifier 212 and zeroes out all of theoffset voltages in the operational amplifiers in the system. FIG. 4Eillustrates the output waveform of the inverting summing amplifier 214.Because of the filtering effect of capacitor C6, the waveform at theoutput of the inverting summing amplifier 214 will become a square waveinstead of a pulse train.

An additional inverting amplifier and filter 216 is used to provideadditional gain and phase inversion to the system so that the polarityof the waveform will the such that the feedback from integrator 218 willcancel out the changes in the flux of the sensor 100. This amplifierstage contains operational amplifier IC6, resistors R16, R17 andcapacitor C7. Because of the capacitor C7, the output of this invertingamplifier 216 will be a triangle wave instead of a square wave. FIG. 4Fshows the output waveform of the inverting amplifier 216.

The integrator with buffer 218 is used to force the input of this stageto a DC zero. The integrator 218 contains resistor R18, capacitor C8,operational amplifier IC7 and a pair of complementary transistors Q3 andQ4. The time constant of this integrator equals R18*C8. This timeconstant is intentionally set very long so that the time it takes toforce the input of the integrator 218 to a DC zero is longer than thetime period over which changes in the input occur, about several hundredmilliseconds. In other words, the long time constant allows the input ofthe integrator 218 to "float" temporarily as it is being forced to zero.Complementary transistor pair Q3 and Q4 is used to provide the currentneeded to counteract the earth's magnetic field and other existingpermanent magnetic fields that are seen in the high permeability corematerial in the sensor 100. Resistor R4 is used to convert the voltageoutput of the integrator 218 into the current needed for cancelling outall of the magnetic flux in the core. If the input to the integrator 218is positive, the output of the integrator 218 will start going lower andreduce or even change the direction of the current flowing into thesecondary coil of sensor 100. This change will force the input of theintegrator 218 to approach a DC zero. If the input to the integrator 218is negative, the output of the integrator 218 will start going higherand increase or even change the direction of the current flow into thesecondary coil of sensor 100. Again, this change will force the input ofthe integrator 218 to approach zero. Thus, the long term DC averagevoltage at the input of the integrator 218 will be zero even though theshort term changes at the input will not be zero. These short termchanges represent the anomalies in the magnetic field.

Low pass filter 222 contains operational amplifier IC8, resistors R19through R22, and capacitors C9, C10. The filter is a preferably a twopole Sallen-Key equal value low pass filter. In the pass band of the lowpass filter 222, the gain is 4.5 db, and in the stop band theattenuation is -40 db per decade. Since the input to the low pass filter222 is the same as the input to the integrator, the long term averageinput value to the low pass filter 222 is zero, and the short termvalue, which represents the magnetic anomaly, is not zero. This filter222 is used to clean up the signal received from the output ofoperational amplifier IC6 of inverting amplifier 216.

An additional low pass filter 224 is used to further clean up the signaloutput from low pass filter 222. The low pass filter 224 containsoperational amplifier IC9, resistors R23 through R26, and capacitorsC11, C12. Like C10 the previous low pass filter 222, this filter 224 ispreferably a two pole Sallen-Key equal value low pass filter. In thepass band of the filter 224, the gain is 4.5 db and in the stop band theattenuation is -40 db per decade. This filter's cutoff frequency is setto be the same as the previous low pass filter 222. The output ofoperation amplifier IC9 in the low pass filter 224 is also the output ofthe sensor amplifier 200. FIG. 4G shows an output waveform of the sensoramplifier 200 when a ferrous metal object is moved near the sensor 100.

When there is no ferrous metal moving near the sensor 100, the wholesensor amplifier 200 will reach equilibrium. Any magnetic motive forceimpressed on the sensor 100 will be cancelled out by a counteractingmagnetic motive force generated by the current flow through resistor 226and the secondary coil of the sensor 100. Thus, the net magnetic fluxflowing through the high permeability core in the sensor 100 will bezero. This equilibrium condition will be maintained regardless of theorientation of the sensor or the presence of nearby stationary ferrousobjects because of the closed loop in the magnetic anomaly detector.However, when a ferrous object is moved quickly near the sensor 100, thesudden change in the magnetic field will cause a momentary change at theinput of integrator 218 (see FIG. 4F) until the integrator 218 has timeto correct the change and bring the circuit back to equilibrium.

For example, assume there is a increase of flux in the core of thesensor 100, resulting in an increase in the positive pulse height and adecrease in the negative pulse height of the pulse train input of theadjustable gain amplifier 210. After amplifier stages 210, 212, 214 and216 there will be a positive DC voltage with a triangle wave at theoutput of inverting amplifier 216. This positive voltage will force theoutput of the integrator 218 to go down. This downward movement of theintegrator output voltage will reduce the current flow through resistor226 and the secondary coil of the sensor 100, counteracting the suddenchange in the magnetic field and forcing the flux in the core of thesensor 100 to become zero. Because of the long time constant used in theintegrator 218, the output of amplifier 216 is allowed momentarily tochange freely. It is this momentary change at the output of amplifier216 that gives the output signal of the sensor amplifier 200.

If there is a decrease of flux in the core of the sensor 100, thischange will result in a decrease of positive pulse height and anincrease of negative pulse height of the pulse train input of theadjustable gain amplifier 210. After amplifier stages 210, 212, 214 and216, there will be a negative DC voltage with the triangle wave at theoutput of amplifier 216. This negative voltage will force the output ofthe integrator 218 to go up. This upward movement of the integratoroutput voltage will increase the current flow through resistor 226 andthe secondary coil of the sensor 100, counteracting the sudden change inthe magnetic field and forcing the flux in the sensor core to becomezero. Because of the long time constant used in the integrator 218, theoutput of amplifier 216 is allowed momentarily to change freely. Again,this momentary change at the output of amplifier 216 gives the outputsignal of the sensor amplifier 200.

Arguably, the output of integrator 218 with AC coupled output and verysmall time constant could also be used as the output representing themagnetic anomaly. However, in order to increase the sensitivity of themagnetic anomaly detector, the resistance of feedback resistor R4 needsto be high, and at large ambient magnetic motive forces the output ofthe integrator simply runs out of room to change at all. One advantageof the present invention's design is that the freedom of movement of theinput signal of the integrator 218 does not depend upon resistance valueof R4, but instead depends upon the time constant R18*C8. Thus, furtheramplification and filtering of the integrator input signal becomes mucheasier.

Referring to FIG. 8, the main amplifier 500 is used to further signalprocess signals from the sensor amplifier 200 and add a sensitivity andthreshold adjustment before feeding the signal into display stage 600.

Adjustable gain amplifier 502 contains operational amplifier IC1,resistors R1, R2, R3 and R5, capacitor C1 and a potentiometer R4connected as a variable resistor. Capacitor C1 and resistors R1, R2block out all the DC voltage so only AC changes can pass through thecircuit. Operational amplifier IC1, along with resistors R3, R4, and R5,are connected as a non-inverting amplifier with a gain adjustment set bypotentiometer R4. Inverting amplifier 503 contains operational amplifierIC2 and resistors R6, R7. During normal conditions, R6=R7, so the gainof the inverting amplifier 503 is -1.

If input 1 of the main amplifier 500 is positive, the adjustable gainamplifier 502 produces a positive signal at the output of operationalamplifier IC1 and a negative signal of equal amplitude at the output ofoperational amplifier IC3. In this case, diode D2 will conduct currentand diode D1 will be reverse biased. Conversely, if input 1 of the mainamplifier 500 is negative, the adjustable gain amplifier 502 produces anegative signal at the output of operational amplifier IC1 and anpositive signal of equal amplitude signal at the output of operationalamplifier IC3. In this case, diode D1 will conduct current and diode D2will be reverse biased. The voltage signal thus produced is thenconverted to a current signal through R15.

Similarly, adjustable gain amplifier 504 contains operational amplifierIC3, resistors R8, R9, R10, R12, capacitor C2 and potentiometer R11 areconnected as a variable resistor. The capacitor C2 and resistors R8, R9block out all of the DC voltage in input 2 so only the AC changes canpass through the circuit. Operational amplifiers IC3, along with R10,R11, R12, are connected as a non-inverting amplifier with gainadjustment set by potentiometer R11. Inverting amplifier 506 containsoperational amplifier IC4 and resistors R13, R14. During normalconditions, R13=R14, so the gain of inverting amplifier 506 stage is -1.

If input 2 of the main amplifier 500 is positive, the adjustable gainamplifier 504 produces a positive signal at the output of operationalamplifier IC3 and a negative signal of equal amplitude at the output ofoperational amplifier IC4. In this case, diode D4 will conduct currentand diode D3 will be reverse biased. Conversely, if the input 2 isnegative, adjustable gain amplifier 504 produces a negative signal atthe output of operational amplifier IC3 and a positive signal of equalamplitude at the output of operational amplifier IC4. In this case diodeD3 will conduct current and diode D4 will be reverse biased. The voltagesignal thus produced is then converted to a current signal throughresistor R15.

Adjustable reference 518 has Zener diode D7, resistor R18, capacitor C3,potentiometer R19 and operational amplifier IC8. This stage is used toset a threshold voltage for cancelling out the effects of the naturalvariations occurring in the earth's magnetic field. The output ofadjustable reference 518 is fed to resistor R18.

Precision diode stage 516 contains operational amplifier IC5, resistorsR15, R16, R17 and diodes D5, D6. When the sensor amplifiers 200 do notdetect any variations in the magnetic field caused by the movement offerrous metal, the changes occurring at input 1 and input 2 will beextremely small. Let us assume for this example that a small ambientmagnetic noise will cause a current flow of I1. If the potentiometer R19is adjusted so that current I2 is equal to or slightly larger than I1,then the output of this stage will be zero. When there is a sudden largechange in the magnetic field, i.e., due to the movement of ferrousmetal, both inputs will fluctuate wildly and current I1 will be largerthan I2, causing a signal to appear at the output of the precision diodestage 516. The resulting output voltage equals (I1-I2)*R17.

Peak detector stage 520 contains operational amplifiers IC6, IC7,resistors R20, R21, R22, capacitor C4 and diodes D8, D9, D10. The peakdetector stage 520 can be any conventional peak detector circuit. Thepurpose of the peak detector 520 is to hold the peak value of therectified signal to make the LED display easier to read; otherwise, theLED display would change too quickly to be read easily. Capacitor C4 isa holding capacitor and resistor R20 is a large value resistor forbleeding the storage charge in the capacitor.

FIG. 9 illustrates the components of display 600. The display 600 servetwo functions. First, it converts the voltage from the main amplifier500 to a bar graph for driving the LED's. Second, it provides anadjusting means for the operator to set how many LED's should be onbefore the buzzer in the system will sound.

Switch assembly 602 contains a single pole double throw switch and apull down resistor. The purpose of the switch assembly 602 is to connecta bar graph display stage 604 either connect to an adjustable referenceamplifier 606 an input signal. The adjustable reference amplifier 606contains operational amplifier IC3 and potentiometer R3. The operationalamplifier IC3 is connected as a unity gain buffer to buffer thereference voltage coming from the potentiometer R3. Normally, 5 volts isused as a reference, but any voltage lower than the 12 volt positivesupply voltage can be used as a reference in this part of the circuit.The output voltage of the reference amplifier 606 is controlled bypotentiometer R3.

Voltage comparator and driver stage 608 contains voltage comparator IC4, resistor R4 , transistor Q1 and an audio buzzer 609. The voltagecomparator IC4 is used to compare the input voltage from the mainamplifier 500 with the reference voltage from the adjustable referenceamplifier 606. If the input voltage of the display 600 is lower than thereference voltage from the adjustable reference amplifier 606, theoutput of voltage comparator IC4 will be low, transistor Q1 will be off,keeping the buzzer 609 mute. If the input voltage of the display 600 ishigher than the reference voltage, the output of voltage comparator IC4will be high. Current will then flow through the current limitingresistor R4, turning on the transistor Q1 thus turning on the buzzer609. The buzzer 609 emits a sound to attract the operator's attention,notifying him or her of the presence of a weapon or other ferrous metalobject passing near the sensor 100.

Bar graph display stage 604 in this example contains resistor R5, bargraph driver IC1 and ten LED's; any number of LED's can be used,however. Bar graph display stage 604 converts an input voltage from mainamplifier 500 to a corresponding number of lit LED's so that the voltagelevel can be displayed visually. If 5 volts is used as the referencevoltage, then any input voltage higher than 5 volts will turn on all tenLED's in the bar graph display. Bar graph display IC1 in this circuitcan be either linear or exponential. Resistor R5 in this circuit is usedto set the brightness of all ten LED's.

Constant current supply 610 contains an adjustable voltage regulatorIC2, resistor R2, and Zener diode D11. The constant current supply 610is used to stabilize the display 600 because without the constantcurrent supply 610, changes in the supply current created by operatingthe buzzer 609 and varying the number of lit LED's will itself generatea changing magnetic field detectable by the sensors 100. This changingmagnetic field will then be fed back to the sensors 100 and cause thewhole system to oscillate. By connecting IC2 and R2 in a constantcurrent supply configuration 610 and using Zener diode D11 to absorb theexcess current not being used by the LED's D1-D10 and the buzzer 609,the display circuit 600 will draw a constant amount of currentregardless of the number of LED's or the buzzer's on/off condition.During normal operating conditions, switch SW1 will be in position A andconnect the bar graph display 604 to the input signal. However, if theoperator wishes to adjust the buzzer sensitivity, the switch SW1 isswitched to position B, and the bar graph display 604 will read thereference voltage from reference amplifier 606 as an input signal. Forexample, when the switch is in position B, the operator may adjustpotentiometer R3. When the switch is returned to position A, the buzzerwill turn on whenever there are more than six LED's lit and give theoperator an audible warning. Whenever there are less than six LED's lit,then the buzzer will remain off.

The embodiment described above uses only two channels, but any number ofchannels may be used to construct the system. One possible configurationplaces one sensor and its corresponding amplifier, the main amplifier,and the display in one box, and a second sensor and its correspondingamplifier in a second box. However, different configurations can be usedto build this magnetic anomaly detector.

Other circuits can be used to perform the same function of sensoramplifier 200. FIGS. 5, 6 and 7 show alternative embodiments for thesensor amplifier construction. For these alternative embodiments, therest of the system is the same as described above.

Referring to FIGS. 5 and 6, alternative sensor amplifier 300 converts aminute change in magnetic flux disturbance caused by the movement offerromagnetic metal near the sensor 100 into a large change of voltageat the output of the sensor amplifier 300. A square oscillator 302contains a 555 timer IC1, resistor R1, and capacitor C1 connectedtogether to generate a near 50% duty cycle square wave. The outputamplitude of the oscillator 302 for this example is 0 to 5 volts.

DC de-coupling stage 303 contains a blocking capacitor C3 and resistorR2. The DC decoupling stage 303 is used to block out the DC voltage inthe output of the oscillator 302 and prevent it from reaching the nextamplifier stage. The output of the DC de-coupling stage 303 in thisexample is a square wave having a magnitude from -2.5 V to +2.5 V.

Driver stage 304 contains an operational amplifier IC2 and a pair ofcomplementary transistors Q1 and Q2. The driver stage 304 is connectedas a unity gain buffer is used to provide the current needed to drivethe sensor 100. A resistor 322 is used to limit the current in sensor100.

Adjustable gain amplifier 310 contains operational amplifier IC3,resistors R4, R5, R7, potentiometer R6, and capacitor C4. The adjustablegain amplifier 310 is used to amplify the output of the sensor 100.Capacitor C4 and resistor R4 separate the AC signal from the DC feedbackthat shared the same winding of sensor 100. Operational amplifier IC3 inthe adjustable gain amplifier 310, along with resistors R5, R7 andpotentiometer R6, is connected as a non-inverting amplifier with gainadjustment capability.

Selective integrator 312 contains diodes D1 through D4, resistors R8through R16 and R18, operational amplifiers IC4, IC5, IC6, capacitor C5,and potentiometer R17. The operation of the selective integrator 312 canbe best explained by referring to FIGS. 6A through 6D, which show theresponse of the selective integrator 312 under different inputconditions.

For this example, it is assumed that the reference voltages are +5 V and-5 V. Also, it is assumed that resistors R8, R9, R11, R12, R14, and R15are all 10K resistors and resistors R10 and R13 are 100K resistors,resistor R18 is 3.3 meg., and capacitor C5 is 0.022 mF. The resistor R16and potentiometer R17 are omitted in FIGS. 6A through 6D for claritybecause they are used in this embodiment only to cancel the offset ofthe operational amplifiers.

Referring to FIG. 6A, the input voltage here is 4.9 V, which is lowerthan +5 V and higher than -5 V, and all the resulting currents and somevoltages are as shown in the schematic. As can be seen in FIG. 6A, theloops having operational amplifiers IC4 and IC5 are closed throughdiodes D1 and D3, and the output of this stage will stay whatever it wasbefore it received the 4.9 V input voltage because there is no chargecurrent through the 0.022 mF capacitor.

Referring now to FIG. 6B, the input voltage in this case is -4.9 V,which is still lower than +5 V and higher than -5 V, and all thecurrents and some voltages are as shown in the schematic. Here, theloops having operational amplifiers IC4 and IC5 are closed throughdiodes D1 and D3, and the output of this stage will stay whatever it wasbefore it received the -4.9 V input voltage because, like the exampleshown in FIG. 6A, there is no charge current through the 0.022 mFcapacitor.

Referring to FIG. 6C, the input voltage here is 5.1 V, which is higherthan +5 V and higher than -5 V, and all the resulting currents and somevoltages are as shown in the schematic. In this case, the loop havingoperational amplifier IC4 is closed through diode D1 and the loop havingoperational amplifier IC5 is closed through the 100K resistor becausethe diode D3 is reverse biased. As a result, the output of the selectiveintegrator 312 in this case will rise at the rate of 100 mA/0.022 mF or4550 V/sec.

Turning now to FIG. 6D, the input voltage in this example is -5.1 V,which is lower than +5 V and lower than -5 V, and all the resultingcurrents and some voltages are shown in the schematic. In this case, theloop having operational amplifier IC5 is closed through diode D3 and theloop having operational amplifier IC4 is closed through the 100Kresistor because the diode D1 is reverse biased. Thus, the output of theselective integrator 312 in this case will fall at the rate of 100mA/0.022 mF or 4550 V/sec.

From the above examples, it is clear that only the portion of the inputvoltage which is higher than the 5 V reference voltage or lower than the-5 V reference voltage will be able to generate a charging ordischarging current through the integrating capacitor C6. Referring backto FIG. 5, operational amplifiers IC4 and IC5 and their associatedresistors and diodes (resistors R8 through R13 and diodes D1 through D4)select which input signals are allowed to pass through to the integrator(resistors R14, R15 and R18 and capacitor C5). One advantage of usingthis selective integrator 312 instead of the circuit shown in FIG. 3 isthat the selective integrator 312 generates a negative voltage referencefrom the positive voltage reference, thus making the circuit symmetricalto the ground. This can eliminate all of the mismatches of the Zenerdiode breakdown voltages. Another advantage of the selective integrator312 is that the operation of this circuit is virtually independent ofthe temperature in the environment. Modern three terminal regulators usea band gap reference diode in the integrated circuit, but band gapreference diodes have extremely small temperature constants compared toZener diodes. By using Zener diodes in a 100% closed loop circuit and asteady reference voltage, the selective integrator 312 can operatewithout any adverse effects caused by temperature dependency. Inaddition, even though the selective integrator 312 uses more componentsthat the selective amplifier 212 of the embodiment shown in FIG. 3, theselective integrator 312 will do the selecting, amplifying, andintegrating of the input signal all in a single stage.

Non-inverting amplifier stage 314 receives the output from the selectiveintegrator 312 and contains an operational amplifier IC7 and resistorsR19, R20. Resistors R19 and R20 are used to set the gain of thisnon-inverting amplifier.

Next, an integrator 316 with a buffer is used to force the input of theintegrator 316 to a DC zero. This integrator 316 contains resistor R21,capacitor C8, operational amplifier IC7 and a pair of complementarytransistors Q3 and Q4. Like the previously described embodiment, thetime constant of this integrator equals R21*C8 and is intentionally setvery long so that the time it takes to force the input of the integrator316 to a DC zero is longer than the time period over which changes inthe input occur, about several hundred milliseconds. The complementarytransistor pair Q3 and Q4 is used to provide the current needed forcounteracting the earth's magnetic field seen in the high permeabilitycore in the sensor 100. A resistor R3 converts the voltage output of theintegrator 316 to a current sufficient to cancel out all the magneticflux in the core of the sensor 100. If the input to the integrator 316is positive, the output of the integrator 316 will start going lower andreduce or even change the direction of the current flowing into thesecondary coil of sensor 100. This change will force the input of theintegrator 316 to approach zero. Similarly, if the input to theintegrator 316 is negative, the output of the integrator 316 will startgoing higher and increase or even change the direction of the currentflow into the secondary coil of sensor 100, again forcing the input ofthe integrator 316 to approach zero. Thus, the long term DC averagevoltage at the input of the integrator 316 will be zero, but the shortterm changes at the input will not be zero. These short term changesrepresent the anomalies in the magnetic field.

Low pass filter 318 contains operational amplifier IC9, resistors R22through R25, and capacitors C7, C8. The low pass filter 318 ispreferably a two pole Sallen-Key equal value low pass filter. In thepass band of the filter 318 of this example, the gain is 4.5 db and inthe stop band, the attenuation is -40 db per decade. Since the input ofthe low pass filter 318 is the same as the input to integrator 316, thelong term average input value to the low pass filter 318 is zero, andthe short term changes which represent the magnetic anomalies is thesignal sought. This filter 318 is used to clean up the signal receivedfrom the output of operational amplifier IC7 in non-inverting amplifier314.

Low pass filter 320 contains operational amplifier IC10, resistors R26through R29, and capacitors C9, C10. This filter 320 is preferably a twopole Sallen-Key equal value low pass filter. In the pass band of thefilter 320 in this example, the gain is 4.5 db and in the stop band, theattenuation is -40 db per decade. In addition, the cutoff frequency ofthe low pass filter 320 is set to be the same as the low pass filter ofthe previous stage 318. This low pass filter 320 is used to furtherclean up the output signal. The output of operational amplifier IC10 inthe low pass filter 320 is the output of the sensor amplifier 300.

When there is no ferrous metal moving near the sensor 100, the wholesensor amplifier circuit 300 will reach equilibrium. Any magnetic motiveforce impressed on the sensor 100 will be cancelled out by acounteracting magnetic motive force generated by the current flowthrough resistor 306 and the secondary coil of the sensor 100. Thus, thenet magnetic flux flowing through the high permeability core in thecenter of the sensor 100 will be zero. This equilibrium condition willbe maintained regardless of the orientation of the sensor axis or thepresence of nearby stationary ferrous objects because of the closedloop. However, when a ferrous object is moved quickly near the sensor100, the change in the magnetic field will cause a momentary change atthe output of amplifier 314 until integrator 316 has the time to correctit and bring the circuit back to equilibrium. Like the embodiment shownin FIG. 3, the time lag for correction causes the output of amplifier314 to "float" momentarily before it is corrected.

For example, assume there is an increase of flux in the core of thesensor 100, resulting in an increase in the positive pulse height and adecrease in the negative pulse height of the pulse train input of theadjustable gain amplifier 310. After amplifier stages 310, 312, 314 and316, there will be a positive DC voltage with a triangle wave at theoutput of amplifier 314. This positive DC voltage will force the outputof the integrator 316 to go down. This downward movement of theintegrator output voltage will reduce the current flow through resistor306 and the secondary coil of the sensor 100, counteracting the effectsof the sudden change and forcing the flux in the core of the sensor 100to become zero. Because of the long time constant used in the integrator316, the output of amplifier 314 is allowed momentarily to changefreely, or "float". It is this momentary change at the output ofamplifier 314 that gives the output signal of the sensor amplifier 300.

Conversely, if there is a decrease of flux in the core of the sensor100, this change will result in a decrease in the positive pulse heightand an increase in the negative pulse height. After amplifier stages310, 312, 314 and 316, there will be a negative DC voltage with thetriangle wave at the output of amplifier 314. This negative DC voltagewill force the output of the integrator 316 to go up, increasing thecurrent flow through resistor 306 and the secondary coil of the sensor100. The increased current flow counteracts the effects of the suddenchange and forcing the flux in the core of the sensor 100 to becomezero. Because of the long time constant used in the integrator 316, theoutput of amplifier 314 is allowed momentarily to change freely. Again,this momentary change at the output of amplifier 314 gives the outputsignal of the sensor amplifier 300. In this embodiment, the freedom ofmovement of the signal does not depend upon the resistance value of 306but instead depends upon the time constant R21*C6. With a groundreference signal, amplification and filtering becomes much easier.

Arguably, the output of integrator 316 with an AC coupled output and avery small time constant could also be used as an output representingthe magnetic anomaly. However, in order to increase the sensitivity ofthe device, the resistance R3 needs to be high, and at large ambientmagnetic motive forces, the output of the integrator may simply run outof room to change. Using the momentary change at the output of amplifier314 overcomes this problem.

FIG. 7 illustrates yet another alternative sensor amplifier 400 for usein the magnetic anomaly detector of the invention.

The sensor amplifier 400 shown in FIG. 7 is very similar to sensoramplifier 200 except that the feedback of the DC current is to theprimary coil of the sensor 100 and not the secondary coil. The feedbacksignal for cancelling out the change in the magnetic flux in the core ofthe sensor 100 is in the form of a voltage to a non-inverting summingamplifier that adds a DC component to the drive signal. This DCcomponent in the drive signal forces a DC current into the primary coilof the sensor 100 to balance out the existing DC magnetic field.

Like the previously described sensor amplifiers 200 and 300, alternativesensor amplifier 400 converts minute changes in magnetic fluxdisturbance caused by the movement of ferromagnetic metal near thesensor 100 into a large change of voltage at the output of the sensoramplifier 400. A square oscillator 402 contains a 555 timer IC1,resistor R1 and capacitor C1 connected together to generate a near 50%duty cycle square wave.

DC de-coupling stage 404 contains of a blocking capacitor C3 andresistor R2. The DC de-coupling stage 404 is used to block out the DCvoltage in the input waveform and prevent it from reaching the nextamplifier stage. The output of DC decoupling stage 404 in this exampleis a square wave that having a magnitude between -2.5 V to +2.5 V.

A driver stage 406 contains an operational amplifier IC3, resistors R3through R6, and a pair of complementary transistors Q1 and Q2. Thedriver stage 406 is connected as a summing amplifier and buffer and isused to sum the square wave and the feedback signal and send the summedsignal through a current amplifier created by transistors Q1 and Q2 todrive the sensor 100. A resistor 408 is used to limit the current insensor 100.

Adjustable gain amplifier 410 contains operational amplifier IC4,resistors R8, R9, R11, potentiometer R10, and capacitor C4. Adjustablegain amplifier 410 is used to amplify the output of the sensor 100.Capacitor C4 and resistor R8 are used for AC coupling the signal outputfrom the sensor 100. Operational amplifier IC4 in the adjustable gainamplifier 410, along with resistors R9, R11 and potentiometer R10, isconnected as a non-inverting amplifier with gain adjustment capability.

Selective amplifier 412 contains Zener diodes ZD1, ZD2, resistors R12through R14, operational amplifier IC5, and capacitor C5 which performthe function of amplifying only the parts of the pulse train that areeither larger than Vr or smaller then -Vr; in other words, the extremetips of the pulse train. Note that Vr is the sum of the breakdownvoltage plus the forward drop of the Zener diode ZD1 and ZD2. Forexample, if the Zener diodes used in the selective amplifier 412 have abreakdown voltage of 5.1 V and a forward drop at low current levels ofapproximately 0.3 V, then the selective amplifier 412 will only amplifythe part of the pulse train which is larger than 5.4 V or lower than-5.4 V. For pulse values within the -5.4/5.4 V range, the output of theselective amplifier 412 simply stays at zero level. Resistors R13 andR14 are used to set the gain of the non-inverting amplifier in theselective amplifier 412 and capacitor C5 is used to provide low passfiltering and a pulse stretching function.

The next stage is an inverting summing amplifier 414. The invertingsumming amplifier 414 contains operational amplifier IC6, resistors R15,R16, R18, capacitor C6 and potentiometer R19. The inverting summingamplifier 414 provides additional gain and filtering to the system.Potentiometer R19 and resistor R16 also compensate for the mismatch ofthe breakdown voltages between the two Zener diodes ZD1 and ZD2 andzeroes out all of the offset voltages in the operational amplifiers inthe system. Because of the filtering effect of the waveform by capacitorC6, the output of the inverting summing amplifier 414 will become asquare wave instead of a pulse train.

An additional inverting amplifier and filter 416 is used to provideadditional gain and phase inversion to the system. This amplifier stage416 contains operational amplifier IC7, resistors R20, R21 and capacitorC7. Because of the capacitor C7, the output of the inverting amplifier416 will be a triangle wave instead of a square wave.

An integrator with buffer 418 is used to force the input of theintegrator 418 to a DC zero. This integrator 418 contains resistor R22,capacitor C8, and operational amplifier IC8. The time constant of theintegrator 418 equals R22*C8 and, like the previously describedembodiments, is intentionally set very long so that the time it takes toforce the input of the integrator 418 to a DC zero is longer than thetime period over which changes in the input occur, about several hundredmilliseconds. The output of integrator stage 418 is fed back throughresistor R4 to create a DC offset needed to cancel out all of themagnetic flux in the core of the sensor 100. Note that the method usedin this circuit is different that the one used in sensor amplifier 200.Instead of sending a DC current to the secondary coil of the sensor, thepresent embodiment forces a DC current through the primary coil of thesensor 100, and the sensor amplifier 400 drives a DC current through thesecondary coil of the sensor 100. If the input to the integrator 418 ispositive, the output of the integrator 418 will start going lower andreduce or even change the direction of the DC current flowing into theprimary core of the sensor 100. This change will force the input of theintegrator 418 to approach zero. If the input to the integrator 418 isnegative, the output of this integrator will start going higher andincrease or even change the direction of the current flow into theprimary coil of the sensor 100. This change will also force the input ofthe integrator 418 to approach zero. Thus, the long term DC averagevoltage at the input of the integrator 418 will be zero, but the shortterm changes at the input will not be zero. These short term changesrepresent the anomalies in the magnetic field.

Low pass filter 420 contains operational amplifier IC9, resistors R23through R26, and capacitors C9, C10. The low pass filter 420 ispreferably a two pole Sallen-Key equal value low pass filter. In thepass band of the filter 420 in this example, the gain is 4.5 db and inthe stop band, the attenuation is -40 db per decade. Like the previouslydescribed embodiments, the long term average input value of the low passfilter 420 is zero, and the short term changes is the signal sought.This filter is used to clean up the signal received from the operationalamplifier IC7 in inverting amplifier 414.

Low pass filter 422 contains operational amplifier IC10, resistors R27through R30, and capacitors C11, C12. This filter 422 is preferably atwo pole Sallen-Key equal value low pass filter. Like low pass filter420, in the pass band of this filter 422, the gain is 4.5 db and in thestop band, the attenuation is -40 db per decade. In other words, thecutoff frequency of the filter 422 is set to be the same as the low passfilter in the previous stage 420. The present low pass filter 422 isused to further clean up the output signal. The output of operationalamplifier IC10 serves as the output of the sensor amplifier 400. As inthe previous embodiments, the output will change when a ferrous metalobject moves near the sensor 100, and the output signal is also fed tothe main amplifier 500 for further signal processing and display.

While the preferred manner for carrying out the invention has beendescribed in detail, those of ordinary skill in the art will recognizevarious alternative designs and embodiments for practicing the inventionwithout departing from the spirit of the invention as defined by theappended claims.

What is claimed is:
 1. An apparatus for detecting anomalies in amagnetic field, comprising:means for generating a square wave; a sensordriven by said square wave generator for sensing changes in the magneticfield of said sensor, said sensor outputting a first pulse train havingan equilibrium value in the absence of anomalies in the magnetic fieldand a pulse height which changes in response to anomalies in themagnetic field; a sensor amplifier connected to the sensor for receivingthe first pulse train from said sensor and for amplifying changes in thefirst pulse train, said sensor amplifier outputting a feedback signal tosaid sensor; said feedback signal representing the difference betweenthe equilibrium value and the changes in the pulse height due to theanomaly in the magnetic field; said sensor amplifier correcting thefirst pulse train to the equilibrium value; a main amplifier connectedto the sensor amplifier for amplifying a filtered waveform output of thesensor amplifier; and a display responsive to the main amplifier.
 2. Themagnetic anomaly detector of claim 1, wherein the sensor amplifiercomprises:a DC decoupling stage for blocking out the DC voltage in thesquare wave generated by the generating means and outputting a decoupledsquare wave; a driver stage for receiving the decoupled square wave andfor providing a driving current to the sensor; a selective amplifier foramplifying portions of the first pulse train which are greater than afirst threshold value or less than a second threshold value, said firstthreshold value being larger than said second threshold value, saidselective amplifier outputting a second pulse train reflecting theportions of said first pulse train which are outside of said first andsecond threshold values; means for converting said second pulse traininto an output waveform whose DC value changes when a magnetic anomalyis detected; an integrator for forcing the average value of the outputwaveform of said converting means to zero over a predetermined timeperiod, wherein the value at the input of the integrator represents themagnetic anomaly, said output of said integrator being sent to saidsensor to compensate for changes in the sensor due to the magneticanomaly, and wherein the predetermined time period is set longer than atime over which short term changes occur in the average value of theoutput waveform of said converting means; at least one low-pass filterfor receiving said waveform from said converting means and outputting afiltered waveform to said main amplifier.
 3. The magnetic anomalydetector of claim 2, wherein said selective amplifier, said convertingmeans, and said integrator are combined into one stage.
 4. The magneticanomaly detector of claim 2, wherein the output of said integrator isconnected to a secondary coil of said sensor for driving the secondarycoil.
 5. The magnetic anomaly detector of claim 2, wherein the output ofsaid integrator is connected to a primary coil of said sensor fordriving the primary coil.
 6. The magnetic anomaly detector of claim 2,wherein said converting means comprises a first and second invertingamplifier for inverting the phase of the second pulse train andconverting it into an output waveform.
 7. The magnetic anomaly detectorof claim 1, wherein said main amplifier comprises:an adjustable gainamplifier for receiving a signal from a selective amplifier and blockingout the DC voltage signal of the signal such that only the AC portion ofthe signal passes through; an inverting amplifier which receives the ACportion of the signal; a precision diode stage which outputs a responsesignal when the output from the sensor amplifier indicates a change inthe magnetic field; and a peak detector for holding the peak value ofthe response signal.
 8. The magnetic anomaly detector of claim 7,wherein the main amplifier further comprises an adjustable referenceacting in conjunction with a summing amplifier for eliminating noisecaused by the earth's magnetic field and sensed by the sensor.
 9. Themagnetic anomaly detector of claim 1, wherein the display comprises:areference amplifier for setting a first reference voltage; a lightingmeans having a plurality of lights responsive to said response signalwherein the number of lights activated is proportional to a ratio of theresponse signal and the first reference voltage; and a sounding meansresponsive to said response signal wherein said sounding means isactivated when said response signal exceeds a second reference voltage.10. The magnetic anomaly detector of claim 9, wherein said firstreference voltage is adjustable.
 11. The magnetic anomaly detector ofclaim 9, wherein said display further comprises means for convertingsaid response signal from said main amplifier into a bar graph display,and wherein said lighting means comprises a bar graph driver and aplurality of LED's responsive to the bar graph driver.
 12. The magneticanomaly detector of claim 9, wherein said display further comprises:aconstant current source; and means for absorbing current from theconstant current source which is not used by said lighting means andsaid sounding means to prevent changes in the magnetic field caused bythe operation of said lighting means and said sounding means.
 13. Anapparatus for detecting anomalies in a magnetic field, comprising:meansfor generating a square wave; a sensor having a core, a primary winding,and a secondary winding, said sensor being driven by said square wavegenerator for sensing changes in the magnetic field of said sensor andoutputting a first pulse train having a equilibrium value in the absenceof anomalies in the magnetic field and a pulse height which changes inresponse to anomalies in the magnetic field; a sensor amplifierconnected to the sensor for receiving the first pulse train from saidsensor and for amplifying changes in the first pulse train, said sensoramplifier having:a DC decoupling stage for blocking out the DC voltagein the square wave generated by the generating means and outputting adecoupled square wave; a driver stage for receiving the decoupled squarewave and for providing a driving current to the sensor; a selectiveamplifier for amplifying portions of the first pulse train which aregreater than a first threshold value or less than a second thresholdvalue, said first threshold value being larger than said secondthreshold value, said selective amplifier outputting a second pulsetrain reflecting the portions of said first pulse train which areoutside of said first and second threshold values; a first and secondinverting amplifier for inverting the phase of the output of said secondpulse train and converting it into an output waveform; an integrator forforcing the DC value of the output waveform of said second invertingamplifier to zero over a predetermined time period, wherein the value atthe input of the integrator represents the magnetic anomaly, said outputof said integrator being sent to one of said primary and secondary coilsof said sensor to compensate for changes in the sensor due to themagnetic anomaly, and wherein the predetermined time period is setlonger than a time period over which short term changes occur in theaverage value of the output waveform of said converting means; and atleast one low-pass filter for receiving said waveform from saidconverting means and outputting a filtered waveform to a main amplifier;said main amplifier connected to the sensor amplifier for amplifying thefiltered waveform output by the sensor amplifier, said main amplifierhaving: an adjustable gain amplifier for receiving a signal from saidselective amplifier and blocking out the DC voltage signal of the signalsuch that only the AC portion of the signal passes through; an invertingamplifier which receives the AC portion of the signal; a precision diodestage which outputs a response signal when the output from the sensoramplifier indicates a change in the magnetic field; an adjustablereference acting in conjunction with a summing amplifier; a peakdetector for holding the peak value of the response signal; and adisplay responsive to the main amplifier.
 14. The magnetic anomalydetector of claim 13, wherein the display comprises:a referenceamplifier for setting a first reference voltage; a lighting means havinga plurality of lights responsive to said response signal wherein thenumber of lights activated is proportional to a ratio of the responsesignal and the first reference voltage; and a sounding means responsiveto said response signal wherein the sounding means is activated whensaid response signal exceeds a second reference voltage.
 15. Themagnetic anomaly detector of claim 14, wherein said display furthercomprises means for converting said response signal from said mainamplifier into a bar graph display, and wherein said lighting meanscomprises a bar graph driver and a plurality of LED's responsive to thebar graph driver.
 16. The magnetic anomaly detector of claim 14, whereinsaid display further comprises:a constant current source; and means forabsorbing current from the constant current source which is not used bysaid lighting means and said sounding means to prevent changes in themagnetic field caused by the operation of said lighting means and saidsounding means.